Hoists have over a number of years in a variety of applications proven to be highly useful and often essential pieces of equipment for performing various necessary industrial and construction operations. However, in spite of the overall excellent safety record which hoists have earned and continue to justify, there is a possibility of inadvertent and unforeseeable failure inherent in any piece of equipment, no matter how well and carefully designed, constructed, maintained and operated. For example, there is a possibility of rope failure.
Since there are certain critical situations where failure of a hoist would be especially serious, the problem of potential failure, even though remote, has received prior consideraion and various solutions have been suggested. For example, U.S. Pat. No. 3,786,935 describes an overhead crane with a single failure-proof hoist that includes a crank hook suspended by a pair of ropefalls. A disadvantage with the device described in this patent is that failure of one of the ropefalls will result in swinging motion of the load since the two ropefalls used to support the load are wound upon opposite ends of the lifting drum.
U.S. Pat. No. 4,069,921 describes a single failure-proof hoist assembly that includes a rotatable drum, drive means for rotating the drum, an upper sheave block, and a lower sheave block supporting a hook assembly. A pair of wire ropes, each capable of fully supporting the load, are wound around the drum and reeved through the upper and lower sheave block and function to support the lower sheave block and the hook assembly in suspended relation. The hoist assembly also includes an equalizer assembly to compensate for variations in the lengths of the two ropes.
U.S. Pat. No. 4,073,476 describes an overhead crane with redundant safety features that includes two independently reeved hoisting systems. Each hoisting system engages one element of a dual element hook and each system is capable of supporting and lifting a first load on the hook. The hoisting systems are adapted to operate in unison to provide redundant safety features when supporting and lifting the first load and, additionally, to support and lift a second load which is greater than the first load. Each independent hoisting system is comprised of: an upper sheave assembly, which includes a plurality of sheaves, mounted on the frame of the crane; a block sheave assembly, which includes a plurality of sheaves, suspended from the frame; at least two ropes reeved on the sheaves of the upper sheave and block sheave assemblies; a take-up means for extending and retracting the ropes, with one of the ends of each rope attachd to the take-up means; and an equalizer bar.
U.S. Pat. No. 4,175,727 describes a hoist with a safety device for uncoupling the prime mover and the high speed, high-kinetic energy components of a drive train from the load following accidental stopping of the load by an external force. A torque-limiting device is provided in the high-speed end of the crane drive which transmits the required running-and static-torques in both directions, but limits the amount of torque which can be imposed on the system by the drive motor and dissipates the kinetic energy of the high-speed end of the drive train when the drive train becomes overloaded. The hoist described in this patent includes diverse-and dual-load paths for supporting critical loads following single failure of the drive train.
U.S. Pat. No. 4,177,973 describes a cable drum safety brake that includes a sensing mechanism which measures the revolutions of both the prime mover and the cable drum. The relationship between the rotation of the prime mover and the rotation of the cable drum corresponds to the speed reduction ratio of the drive train in normal operation. A detection circuit examines the rotation of the prime mover and cable drum for a deviation from this relationship and actuates a cable drum braking mechanism in response thereto to stop the rotation of the cable drum.
Federal regulations require, among other things, that the design of the rope reeving systems for overhead cranes for use in nuclear facilities be dual with each system providing separately the load balance on the head and load blocks through configuration of ropes and rope equalizers.